Zinc-copper-tin alloy plating



Patented June 17, 1952 ZINC-COPPER-TIN ALLOY PLATING Roy E. Shockley, Kokomo, Ind., assignor to R. E.

Shockley, Incorporated, Kokomo, Ind., a corporation of Indiana No Drawing. Application October--19, 1950. Serial No. 191,089 i 7 Claims. 1

The present invention relates to a plating method for depositing, upon a platable cathode, a coating of high density, capable of taking a high polish, and highly resistant to corrosion and abrasion, such coating comprising an alloy consisting of zinc, copper and tin in suitable proportions. The invention relates, also to an electrolyte essential to the practice of the above method.

The primary object of the invention is to provide a method, and an electrolyte for use therein, whereby through the use of an anode composed of a suitable alloy of zinc, copper and tin, such alloy may be electro-deposited upon a suitable cathode, in substantially the proportions in which the ingredients appear in the anode. A further object oi. the invention is to provide and maintain, during the plating process, an electrolytic bath of such character as to support the electro-deposition of such alloy upon the cathode.

Still further objects of the invention will appear as the description proceeds.

To the accomplishment of the above and related objects, my invention may be embodied in the forms, and in the steps, described in the following specification, attention being called to the fact, however, that the specific embodiments described are by way of illustration only and that changes may be made therein so long as the scope of the appended claims is not violated. I have discovered that an alloy, consistingof Zinc approximately 90%, copper approximately 8.5% and tin approximately 1.5% has many highly desirable physical characteristics. Such an alloy has a density of approximately 7.24, a color, when unpolished, closely similar to that of cast aluminum, is capable of taking a high polish, has a ductility comparable to that of conventional copper or tin plate, is highly resistant to corrosion by substances encountered in nature, such as sea water and the like, resists ordinary abrasion, is weldable by ordinary methods, and is compatible with commercial types of solders and brazing metals. Additionally, I have found that, under suitable procedures, the alloy may be electro-deposited, substantially without change of proportion am ng its ingredi n at hi h efiioiencies. When so deposited, under proper conditions, the plate retains the above characteristics and, when the plating process is properly controlled, the deposit will be found to possess a high metallic luster, without polishing or buffing, or with a minimum thereof.

I have found that a bath or electrolyte comprising an aqueous solution of suitable proportions of suitable salts of the three metals embodied in this alloy, together with a suitable proportion of free cyanide, and preferably including also a substantial amount of a caustic alkali, is essential to the practice of my method. Preferably the zinc and copper will be introduced into the bath in the form of metal cyanides, while the tin is preferably introduced in the form of potassium stannate.

Experimentation has established the fact that the rate of corrosion of the metals from the anode may be to a considerable degree individually controlled by variations in the relative proportions in which free cyanide and caustic alkali are present in the electrolyte. Broadly speaking, an increase in the ratio of cyanide radical to caustic alkali in the electrolyte will result in an increase in the rate of corrosion of zinc and copper from the anode; while a decrease in that ratio willresult in an increase in the rate of corrosion of tin from the anode. Conveniently, the free cyanide may be introduced into the bath as sodium cyanide, though any of the alkali cyanides may be used; while sodium hydroxide will preferably be used as the caustic alkali.

An optimum electrolyte may be prepared by dissolving, in one gallon of water,

Ounces NaOH 9 NaCN 13 Zn(CN)'2 8.75 CllCN 1 K2SI103'3H2O 0.15

While the above proportions are optimum, according to my present belief, variations in the proportions of the ingredients, within the following ranges are permissible without substantial effect upon the functioning of the electrolyte or upon the end product.

Ounces NaOH 6.5-9 1 NaCN 3-13.,5 Zn(CN)2 6.5-11 CuCN 0.5-2 K2SnO3-3H2O 0.1-0.3

After the bath has been prepared, the cathode piece or pieces should be thoroughly cleaned, preferably in alkaline electrolytic cleaner such as a 4% good commercial cleaner compound at a boiling temperature. If steel parts are to be plated, it is preferable to subject them to a' resimilarly suspended in the bath. -Eacha'n'ode'w'ill preferably present a fiat face toward each of the pair of cathodes associated therewith, thecath odes being preferably spaced approximately five inches from such faces.

Each anode may be a cast her of the alloy above defined, comprisingap proximately 90% zinc, 8.5% copper and 1.5% tin.-

A plating current of to 45 amperes per square foot, at a potential of 3 to 4 volts, is now applied between .the anode and the cathodes, .a density of -to 37.5 amperes per square foot, depending upon cathode shape, being optimum. 'I have found that the potential will sometimes rise above 4 voltsin the early stages of the process; butthat, aft-era few minutes, it will drop to a value of 3 /2 to 4 volts where it will continue quite steadily throughout the plating operation. Under such conditions, a deposit of 0.000l'inch thickness will be formed upon the cathode' evei-y 3 to 4 minutes; and I believe this deposit 'rate'to be opti-- mum. My best results have been attained through the use of a potential of 3.8volts at a density of amperesper square foot of cathode surface. I

The temperature ofthe bath should preferably be held between and F.; though my experiments indicate that the process will not be substantially affected if the temperatureis permitted to drop as low as 70 or to rise as high as I After the desired thickness of deposit has been formed on the cathodes, they should be removed from the bath, rinsed in boiling water-and then suitably dried. It will be found that a bright,

white deposit, closely resembling a good cadmium plate in appearance and physical characteristics, will have been formed.

The platable metal-content of the bath (zinc, copper and tin) should preferably be present substantially in the proportions in which those metals appear in the anode; viz., zinc 90%,c'op= .per 85% and tin 1.5%. It will be seen that the bath, as originally prepared in accordance with the above instructions, contains zinc 86.35%,

copper'l2;59% and tin 1.06%. When the plating -current is applied between the-defined anode and a suitable'cathode for a period of, for instance, twelve hours, it is found that the composition of the bath, as concerns the platable metals, will have been changed to contain zinc 5.51 01111869401 90.33%, copper 0.50 ounces, or 8.20% and tin 0.05 .ounces or 1.47%, per gallon of solution. This is a 1e 's'ult, of course, of differing rates of corrosion of the" respective metals from the anode into the solution, as related 'to'the" rates of" deposition of the respective'rnetals upon the cathode. 'It'will be noted that, in the'bath as originall prepared, the cyanide'radical, obtained as'sodi- =um'cyanide, is present in the amount of 20.85 ounces per gallon, while platable metals are pres- ."jent to the amount of 5.64 ounces per gallon. .;Thus, as originally prepared, the-ratio of cyanide, obtained as sodium cyanide, to total ,platable 2,600,699 31' "II N however, it will be found that total cyanide in the bath, obtained as sodium cyanide, will be 12.7 ounces per gallon; so that the ratio of cyanide, obtained as sodium cyanide, to total platable metals, will now be 2.08.

I presently believe that control of the ratio of total cyanide to total platable metals, in the bath, within a range from 1.90 to 3.75 is desirable in order to produce optimum'results; and that a ratio close to 2.50 should be maintained durin continuous plating operation. Thus, to the bath ".of the composition above described after twelve hours of operation, Iprefer to add 2.55 ounces of sodium cyanide 'per 'gallOn of electrolyte, or an equivalent mass of some other alkali cyanide.

If the 'cyanide-to-metals ratio rises substan- 'tiallyabove 2.5, during operation, the ratio may be restored to approximately the optimum value by the addition of sodium hydroxide, or some other caustic alkali, in quantities sufiicient to increase the rate at which the anode metals will be attacked and corroded or dissolvedin the'ele'ctrolyte.

' Under conditions as above described, the efficiency of the present 'plating'method' is'unusually high. Forinstance, during a 'twelve-hourtest under such conditions, the loss 'of'weight in the anode was 13.38 grams, while the gain of weight in the cathode was 13.82 grams representing a plating efficiency of approximately 96.82%."

It is to be" noted ".that'the "cyanide to -m'etals ratio preferably maintained in'the bath is lower than that conventionally considered toibe 'desir able in cyanideelectrolytes. 'The'recommended ratio, however, is found, in'the method of the present application, to produce a higher rate of deposition, and to result in a brighter and better color than can be obtained with higher'ratios.

Sometimes, after extended periods of operation, the plating solution is found 'to become opalescent and dark. 'This condition'canzbe rectified either by reversing the currentlflow, using a steel anode, or'by the addition .of a very'small amount of hydrogen peroxide onlthe'orderof two drops of 30% hydrogenperoxide .to. approximately five liters of electrolyte.

The resistance of the plated deposit to.corro sion has beendetermined by carefully .co'n'ducted tests in which strips platedaccording to .thepresent process were-subjected toisalt spray. "Under those tests, a strip carrying a deposit 0.0003? thick failed after 1466'hours, 'while similarly plated strips carrying deposited '.thicknesses..of

0.0004" and 0.0005" Werenotnompletelyibroken after 1466 hours of -subjection to .the saltspray. Instill another test, a similarstrip plated-to the thickness of 0.00015 was-subjectedto thesalt spray, and it was found that-thefirst break-occurred after 144 hours of such exposura I claim as my invention:

1. The method of producing a corrosion-resistant plated surfaceon a platable articlezwhich comprises the stepsofimmersing in a'bathscomprising an aqueous solution ofzzinc, copper-and tin salts,"and containing an alkali cyanide and-a caustic alkali, a'-'cathode"to beplate'd and an anode comprising an alloy of zinc'appro'ximatel'y ibi tw nqd J i .1

2. The method of claim 1 in which, per gallon of water, the dissolved substances are:

Ounces Sodium hydroxide 6.5-9 Sodium cyanide u 3-135 Zinc cyanide 6.5-11 Cuprous cyanide 0.5-2 Potassium stannate 0.1-0.3

3. The method of claim 1 in which, per gallon of water, the dissolved substances are:

Ounces Sodium hydro 8 Sodium cyanide 4.07 Zinc cyan 9.89 Cuprous cyanide 0.70 Potassium stannate 0.23

4. The method of claim 1 in which, per gallon of water, the dissolved salts of metals contained in the anode are:

Ounces Zinc cyanide 6.5-11 Cuprous cyanide 0.5-2

Potassium stannate -1 0.10.3

5. The method of claim 1 in which the current density is maintained in the range between and amperes per square foot of immersed cathode surface.

6. The method of claim 1 in which the temperature of the bath is maintained in the range between and F.

7. The method of claim 1 in which the plating current is maintained approximately at 3.3 volts and 35 amperes per square foot of immersed cathode surface, and the temperature of the bath is held between 70 and 140 F.

ROY E. SHOCKLEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,079,842 Cinamon May 11, 1937 2,435,967 Jernstedt Feb. 17, 1948 2,530,967 Jernstedt Nov. 21, 1950 

1. THE METHOD OF PRODUCING A CORROSION-RESISTANT PLATED SURFACE ON A PLATABLE ARTICLE WHICH COMPRISES THE STEPS OF IMMERSING IN A BATH COMPRISING AN AQUEOUS SOLUTION OF ZINC, COPPER AND TIN SALTS, AND CONTAINING AN ALKALI CYANIDE AND A CAUSTIC ALKALI, A CATHODE TO BE PLATED AND AN ANODE COMPRISING AN ALLOY OF ZINC APPROZIMATELY 90%, COPPER APRROXIMATELY 8.5% AND TIN APPROXIMATELY 1.5%, AND PASSING THROUGH SAID BATH BETWEEN SAID ELECTRODES, A PLATING CURRENT THE METAL COMPONENTS OF SAID SALTS BEING PRESENT IN THE BATH SUBSTANTIALLY IN THE PROPORTIONS IN WHICH THE CORRESPONDING METALS ARE PRESENT IN THE SAID ANODE. 